1. Field of the Invention
This invention relates to an air flow rate measuring apparatus used especially for the measurement of the flow rate of air taken into an internal combustion engine.
2. Description of the Prior Art
An example of the conventional air flow rate measuring apparatus is disclosed in, for example, the U.S. patent application No. 3,824,966, in which a thin bare wire of metal is extended across the main venturi so as to be used as a flow sensor.
Such a flow sensor is exposed directly to the adverse influence of the abnormal condition such as the back fire of engine so that the fine wire tends to be damaged by thermal and mechanical impacts due to the abnormal phenomenon. Moreover, since the sensor is placed in the main venturi where the rate of flow of the intake air is very large, microscopic dust suspended in the intake air tends to be deposited on the fine wire. The deposited dust deteriorates the characteristic of the sensor. Further, the above cited flow sensor is subject to thermal disturbance due to heat conduction through the supporting member of, for example, metal and the disturbance leads to error in the measurement of air flow.
Another example of the conventional air flow apparatus is disclosed in, for example, the Japanese Patent Laid-Open No. 113432/77. According to the cited reference, the air flow meter is located in a bypass provided apart from the main venturi. In this reference, however, no thermal influence on the air flow meter is considered.